Use has been made of a computerized numerically controlled lathe to cut workpieces into a desired shape. The lathe usually includes, among other components, a spindle with a chuck for holding workpieces to be cut and an electric drive motor drivingly connected to the spindle for causing the spindle to rotate at a preselected speed. In the course of turning operation, the torque of the spindle has to be increased or decreased depending on the cutting load. This is because the spindle requires greater torque in case of heavy cutting than needed for light cutting. The spindle torque can be changed through the use of a lathe transmission provided in between the electric motor and the spindle.
A typical example of commercially available lathe transmissions is shown in FIG. 1, which comprises a casing 10 and a couple of spaced-apart, parallel input and output shafts 12, 14 each journalled on the casing 10 with bearings 16. Affixed to the external end of the input shaft 12 is an input pully 18 which in turn is operatively associated with and rotatingly driven by an electric motor not shown for simplicity. An output pully 20 is fixedly secured to the external end of the output shaft 14 for unitary rotation therewith. The output pully 20 is in operative association with a spindle not shown.
First and second stationary gears 22, 24 are attached to the output shaft 14 in a manner that they cannot move in the axial direction of the output shaft 14. The first stationary gear 22 has a diameter smaller than that of the second stationary gear 24. A movable gear 26 with first and second gear sections 26a, 26b is axially slidably keyed to the input shaft 12 for sliding movement between a first shift position wherein the first gear section 26a of the movable gear 26 comes into meshing engagement with the first stationary gear 22 to attain reduced torque and increased rotation speed of the output shaft 14 and a second shift position wherein the second gear section 26b of the movable gear 26 is meshed with the second stationary gear 24 to accomplish increased torque and reduced rotating speed of the output shaft 14.
The sliding movement of the movable gear 26 into the first and second shift positions is effected by virtue of a gear shifting device generally designated at 30. The shifting device 30 includes a double acting shift cylinder 32 which is composed of a cylinder housing 34 mounted to the transmission casing 10 and a cylinder rod 36 slidably fitted to the cylinder housing 34. The cylinder housing 34 has a first pressure chamber 34a and a second pressure chamber 34b respectively communicating with a fluid pump(not shown) via inlet and outlet ports 34c and 34d.
The cylinder rod 36 is provided at it's internal end with a shift yoke 38 of the type remaining engaged with the movable gear 26 at one extremity thereof and slidably fitted to a guide bar 40 at the other extremity. At the external end of the cylinder rod 36, a dog 41 is provided to activate either a first proximity switch 42 to thereby stop the extending movement of the cylinder rod 36 or a second proximity switch 44 to thereby stop the retracting movement of the cylinder rod 36. Extending the cylinder rod 36 will enable the shift yoke 38 to bring the movable gear 26 into the first shift position so that the first gear section 26a of the movable gear 26 can mesh with the first stationary gear 22, whereas retracting the cylinder rod 36 will enable the shift yoke 38 to bring the movable gear 26 into the second position, thus allowing the second gear section 26b of the movable gear 26 to mesh with the second stationary gear 24.
With the conventional lathe transmission as referred to hereinabove, the shift yoke is subjected to severe wear because it continues to be in frictional contact with the rotating movable gear. Such wear may result in misalignment of the movable gear with respect to the stationary gears in the first and second shift positions. To assure exact gear alignment, the worn-out shift yoke should be replaced with a new one through a laborious replacement operation and at an increased cost. Moreover, since the shift yoke is mounted on the internal cantilevered end of the cylinder rod, a great magnitude of bending moment exerts on the cylinder rod during and after the shifting operation, which may lead to flexural deformation of the rod and leakage of the working fluid out of the cylinder housing. In addition to these deficiencies, the prior art lathe transmission is disadvantageous in that the cylinder housing of the shift cylinder must be manufactured apart from and then mounted to the transmission casing and further that it is unavoidable to use the guide bar in order to prevent unwanted rotation of the shift yoke and the cylinder rod about the axis of the latter. As a solution to the drawbacks noted above, need has existed for a lathe transmission whose gear shifting device has an enhanced structural integrity, requires a reduced number of parts and is manufacturable in a cost-effective manner.